In compression molding a preform for subsequent blow molding as a plastic container, a mold charge is fed to a female compression mold cavity and a male mold core is inserted into the cavity to form the mold charge into a preform. This preform is subsequently blow molded to form a plastic container. The mold charge typically is fed to the mold cavity using a transfer wheel or the like that obtains a mold charge from the outlet of an extruder or other melt-phase transfer mechanism, and then releases the mold charge to fall by gravity into the female mold cavity. This transfer wheel and gravity drop technique is not sufficiently fast for high-speed production of preforms and containers, and does not consistently result in the desired placement and orientation of the mold charge within the mold cavity. It is a general object of the present invention to provide an apparatus and a method that reduce mold charge transfer time as compared with the prior art, and increase the accuracy of placement and orientation of the mold charge within the mold cavity.
An apparatus for supplying a mold charge for a mold cavity in accordance with one aspect of the present invention includes a mold charge chamber having a longitudinal dimension and at least two lateral inlets for supplying resin materials to the chamber such that the materials are stratified longitudinally of the chamber. A plunger is movable longitudinally into the chamber to force the stratified materials out of the chamber and thereby provide a stratified mold charge for a mold cavity. The at last two lateral inlets to the chamber preferably are spaced from each other lengthwise of the chamber. In one embodiment, the plunger has a contoured end for imparting a contour to the mold charge as the mold charge is forced from the chamber. A gate preferably is positioned at an end of the chamber remote from the plunger for selectively closing the chamber for filling and opening the chamber for discharge of the mold charge.
Apparatus for compression molding preforms for blow molding plastic containers in accordance with another aspect of the present invention includes a mold charge chamber having a longitudinal dimension and a lateral dimension that is uniform throughout the length of the chamber. At least one lateral input to the chamber is connected to a source of resin material to fill the chamber. A plunger has a lateral dimension substantially equal to the lateral dimension of the chamber, and is movable between a first position that closes an upper end of the chamber and a second position in which the plunger substantially fills the chamber. A gate is positioned at an end of the chamber remote from the plunger for selectively closing the chamber for filling and opening the chamber for discharge of a mold charge as the plunger moves toward the second position. A preform mold cavity is disposed to receive a mold charge discharged from the mold chamber, and a mold core is positioned to enter the mold cavity and compression mold a container preform. In one preferred embodiment of the invention, the at least one lateral inlet to the chamber includes at least two lateral inlets that are spaced from each other longitudinally of the chamber. The at least two inlets are connected to associated sources of plastic resin material for supplying resin to the chamber in such a way that the materials are stratified longitudinally of the chamber. The mold charge supplied to the mold cavity is thus stratified longitudinally of the mold charge, and is adapted for compression molding preforms having layers of material corresponding to the material stratification in the mold charge.